On 7/16/21 4:14 PM, Sean Christopherson wrote:
On Wed, Jul 07, 2021, Brijesh Singh wrote:
+static unsigned long snp_handle_psc(struct vcpu_svm *svm, struct ghcb *ghcb)
+{
+ struct kvm_vcpu *vcpu = &svm->vcpu;
+ int level, op, rc = PSC_UNDEF_ERR;
+ struct snp_psc_desc *info;
+ struct psc_entry *entry;
+ gpa_t gpa;
+
+ if (!sev_snp_guest(vcpu->kvm))
+ goto out;
+
+ if (!setup_vmgexit_scratch(svm, true, sizeof(ghcb->save.sw_scratch))) {
+ pr_err("vmgexit: scratch area is not setup.\n");
+ rc = PSC_INVALID_HDR;
+ goto out;
+ }
+
+ info = (struct snp_psc_desc *)svm->ghcb_sa;
+ entry = &info->entries[info->hdr.cur_entry];
Grabbing "entry" here is unnecessary and confusing.
Noted.
+
+ if ((info->hdr.cur_entry >= VMGEXIT_PSC_MAX_ENTRY) ||
+ (info->hdr.end_entry >= VMGEXIT_PSC_MAX_ENTRY) ||
+ (info->hdr.cur_entry > info->hdr.end_entry)) {
There's a TOCTOU bug here if the guest uses the GHCB instead of a scratch area.
If the guest uses the scratch area, then KVM makes a full copy into kernel memory.
But if the guest uses the GHCB, then KVM maps the GHCB into kernel address space
but doesn't make a full copy, i.e. the guest can modify the data while it's being
processed by KVM.
Sure, I can make a full copy of the page-state change buffer.
IIRC, Peter and I discussed the sketchiness of the GHCB mapping offline a few
times, but determined that there were no existing SEV-ES bugs because the guest
could only submarine its own emulation request. But here, it could coerce KVM
into running off the end of a buffer.
I think you can get away with capturing cur_entry/end_entry locally, though
copying the GHCB would be more robust. That would also make the code a bit
prettier, e.g.
cur = info->hdr.cur_entry;
end = info->hdr.end_entry;
+ rc = PSC_INVALID_ENTRY;
+ goto out;
+ }
+
+ while (info->hdr.cur_entry <= info->hdr.end_entry) {
Make this a for loop?
Sure, I can use the for loop. IIRC, in previous review feedbacks I got
the feeling that while() was preferred in the part1 so I used the
similar approach here.
for ( ; cur_entry < end_entry; cur_entry++)
+ entry = &info->entries[info->hdr.cur_entry];
Does this need array_index_nospec() treatment?
I don't think so.
thanks
